This invention relates to the use of acidophilic bacteria, such as Thiobacillus ferrooxidans to regenerate chelated polyvalent catalysts (chelating agents) used in the oxidation of hydrogen sulfide to elemental sulfur for enhanced economics in sweetening of sour natural gas. More particularly, the invention relates to biologically enhancing the production of elemental sulfur and enhancing the thermal stability of the costly chelating agents during the operation of a hydrogen sulfide removal process in which said catalysts are regenerated.
Treatment of Sour Natural Gas
The natural gas industry has long been interested in sulfur recovery technology for applications to gaseous streams resulting from the treatment of sour natural gas resources to render them commercially useful. Many natural gas resources contain significant quantities of hydrogen sulfide (H.sub.2 S) and other contaminants. Such "sour" gas is hazardous to human health and could cause extensive damage to natural gas pipelines if not properly processed. In order to reduce health and environmental hazards, and to meet the pipeline industry specifications, the H.sub.2 S concentrations in natural gas are ordinarily reduced to less than 4 parts per million in volume (ppmv). About twenty five percent of the natural gas produced in the United States contains significant volumes of H.sub.2 S and other sulfur compounds.
A traditional process for treating this sour gas is the Amine-Claus process which involves a two-step approach of first separating the acidic gases from the natural gas in an Amine plant and then either flaring the hydrogen sulfide off or recovering the sulfur in a separate Claus plant. Liquid redox processes, such as the Stretford process, are commonly preferred over the Amine-Claus systems because of their greater simplicity, higher sulfur recovery and good turndown ratio. Exemplary references more particularly describing these matters include U.S. Pat. Nos. 4,009,251; 4,243,648; and 3,937,795. Scavenger processes are preferred for natural gas streams where sulfur recovery is not economical.
Liquid redox sulfur recovery processes absorb hydrogen sulfide from the sour gas stream, ultimately producing elemental sulfur. Liquid redox processes may use, for example, vanadium, iron or a mixture of iron and quinone as the primary catalysts interacting with hydrogen sulfide.
The Stretford liquid redox process uses vanadium as the catalyst. Unfortunately, vanadium is toxic at any concentration and environmental regulations prevent its disposal at concentrations above 25 ppm. On the other hand, iron based catalyst systems are generally preferable because of their non-toxic, generally environmentally friendly character. The iron-based catalyst systems have been successful because of their superior performance, simple operation, greater reliability, as well as their environmental acceptability. A recognized drawback to the iron-based systems is that the commercial processing conditions promote oxidation reactions and thereby accelerate the decomposition of the metal-chelate catalysts essential for the reaction, resulting in undesirably high processing costs, including increasing recirculation power requirements. Moreover, in all the commercial liquid redox processes, expensive redox solution is inevitably lost via salt formation and unrecoverable residue in the sulfur cake resulting from the process.
In the late 1970's, ARI Technologies of Palatine, Ill., developed a liquid redox process under the trade name LO-CAT.RTM., which has undergone a number of refinements aimed at improved reliability and economics. In 1987, Shell/DOW introduced a liquid redox process under the trade name SulFerox.RTM., which uses a scrubbing solution containing 2 to 4 weight percent of iron. This process provides relatively high H.sub.2 S removal capacity for a given volume of solution.
Iron based redox processes employ iron in the ferric state (Fe.sup.3+) to oxidize hydrogen sulfide in a stream of sour gas to elemental sulfur (S.sup.0), whereby the ferric iron is reduced to the ferrous state (Fe.sup.2+), which is then regenerated to the ferric state by oxidation with air as follows: ##EQU1##
Typical iron concentrations range from 500-2500 ppm in the catalyst. Concentrations are varied according to economics of pumping and chemical costs attributable to the specialized circumstances of particular applications and processing facilities.
Neither ferric nor ferrous ions are stable in aqueous solutions at neutral or alkaline pH levels and will, therefore, ordinarily precipitate as either ferric or ferrous hydroxide. This precipitation is inhibited by complexing the iron with organic chelates which are capable of holding both Fe.sup.2+ and Fe.sup.3+ ions in solution over the wide range of pH typically encountered during commercial processing.
The organic chelates utilized in these redox systems are ordinarily classified into two groups: type A chelates such as ethylenediamine tetraacetic acid (EDTA) or nitrilotriacetic acid (NTA), powerful chelating agents at low pH; and the type B chelates, consisting of polyhydroxylated sugars (saccharides) that are effective at pH above 8. A proprietary combination of both types of chelates results in a catalyst that is stable over a range of pH, for example, from 5 to 9.0.
In work with the LO CAT process, McManus disclosed, in U.S. Pat. No. 4,622,212 (incorporated herein by reference), the desirability of combining an aminopolycarboxylic acid chelating agent (type A) with a polyhydroxylated saccharide (type B) chelating agent. McManus observed that chemical degradation and loss of the aminopolycarboxylic acid chelating agent, thereby necessitating addition of replacement chelating agent is the single most significant operating problem affecting the ultimate economic feasibility of prolonged large-scale operation of this liquid redox process. McManus and others have suggested the use of additional stabilizing agents or additives to the catalyst system, such as alkaline thiosulfate, t-butanol and ethylene glycol. According to the literature, even in the presence of such stabilizing agents, only limited stability of the commercial chelating agents at processing temperatures in excess of 35.degree. C. has been attained.
The aminopolycarboxylic acid chelants such as nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA), N-hydroxyethylenediamine tetraacetic acid (HEEDTA), and diethylenetriamine pentaacetic acid (DTPA) are powerful chelating agents at low pH. The polyhydroxylated saccharide chelants, such as sorbitol, are effective at pH above 8.0.
The combination of multiple chelants selected from both groups can result in a catalyst system that is stable over a wide range of pH, from 2 to 10.0.
Liquid Redox Process Degeneration Products in Iron Based Redox Processes
The selection of particular chelants is dependent on the reaction rate of (1) the Fe.sup.3+ chelate with H.sub.2 S, (2) the reaction rate of the Fe.sup.2+ /Fe.sup.3+ chelate with oxygen, and (3) the rate of degradation of the chelate. Ferric ion oxidation of the chelate can be controlled by maintaining the overall reaction temperature below 45.degree. C. Chelate degradation occurs through the oxidation of chelate by Fe.sup.3+ ion and free radical induced oxidation. Other variables that control the oxidative degradation are pH, chelate concentration, chelate to iron ratio, and the type of degradation products formed under process conditions.
The LO-CAT.RTM. process was originally developed by ARI Technologies, now Wheelabrator Clean Air Systems, Inc. (WCAS), to treat sour gas in an absorber vessel where the absorption of the H.sub.2 S and oxidation to sulfur takes place and a reoxidation where the chelated iron is reactivated by oxidation by exposure to air in a stirred reaction chamber. This system, referred to as "conventional" LO-CAT, works well for many low-pressure plants at feed gas pressure and relatively low iron concentrations (1000 to 1500 ppmw) and high circulation rates. This system requires prohibitively expensive equipment and pumping costs for high pressure applications, however.
Modifications to the original LO-CAT.RTM. process, referred to as the ARI LO-CAT II process (shown in the FIG. 1 process flow diagram) were developed for high pressure "direct treat" applications. The LO-CAT II.RTM. process uses substoichiometric iron chelated catalysts in the absorber and an oxidizer unit that circulates liquid through density differences. This "staged" oxidation of the LO-CAT II system circulates throughout the solution by means of density differences generated by controlled aeration, such as by sparging, with oxidizing air, rather than by a mechanically well-stirred reaction chamber (WSTR). Solutions are withdrawn from the oxidizer at the last, or most oxidized stage. This delayed withdrawal or "draw down" involves the overall oxygen mass transfer and thereby permits the design of smaller oxidizers. Ratios of iron to H.sub.2 S as low as 20% of the stoichiomatic requirement have been reported as successfully utilized. The process also uses a separate sulfur settler vessel. These features reduce both the chemical and operating costs.
Mechanism of Microbial Oxidation of Ferrous Iron
Thiobacillus ferrooxidans, an iron oxidizing bacterium discovered in 1947, has been used since 1984 in Japan to regenerate an iron based sour gas treating solution without catalysts, and without side reactions such as formation of thiosulfuric acid, at an acidic pH of about 2, as described by H. Satoh, et al., of the NKK Corporation in Hydrocarbon Processing, May, 1988, incorporated herein by reference.
The iron oxidizing bacteria are, by definition, capable of oxidizing ferrous (Fe.sup.2+) ions to the ferric (Fe.sup.3+) ion state at low pH. According to the literature, such bacteria are capable of oxidizing Fe.sup.2 + to the Fe.sup.3+ state at a rate about 500,000 times faster than in a non-biologically mediated chemical oxidation process in the absence of bacteria. In theory, these bacteria derive the energy required for their growth from the oxidation of reduced sulfur compounds and the oxidation of Fe.sup.2 + to Fe.sup.3+, using air as an oxidant.
The acidophilic iron bacteria, Thiobacillus ferrooxidans, generates ATP by a membrane bound ATP catalase. This ATP generating metabolism is driven by the proton motive force derived by the difference between the bacterium's neutral cytoplasm and its highly acidic environment to generate a transmeinbrane proton electrochemical potential. Neutralization of the bacterium's cytoplasm is catalyzed by the cytochrome oxidase reaction. The regeneration of Fe.sup.3+ chelate in the presence of acidophilic microbes such as Thiobacillus ferrooxidans under mild conditions at 25-45.degree. C., and atmospheric pressure minimizes the chelate degradation process and thus improves the economics of hydrogen sulfide oxidation in the commercial natural gas sweetening process. Another useful feature is that the regenerated Fe.sup.3+ -chelates are also capable of oxidizing the mercaptans to insoluble disulfides.
There is evidence that Thiobacillus ferrooxidans may not be one distinct bacterium, but rather a group of metabolically similar microbes. It has long been recognized that sulfur sequestration or removal is carried out by a variety of bacteria. Acidophilic bacteria that grow as heterotrophs include Acidiphilum cryptum and Thiobacillus acidophilus, while those that grow as autotrophs include Thiobacillus ferrooxidans, Thiobacillus thiooxidans and Leptospirillum ferrooxidans. Non-acidophilic bacteria that grow in sulfur containing media, usually in the presence of glucose as an energy source include species of Pseudomonas, Escherichia coli, and Thiobacillus novellus. The recognized optimum pH for growth of T. ferrooxidans on ferrous ion is about 2.0. The oxidation of ferrous (Fe II) to ferric (Fe III) thus occurs outside the cell wall, where such low pH would not be fatal to the cell, whereas reduction of oxygen occurs inside the cell membrane at a biologically acceptable pH of about 6.5. Cytochrome oxidase mediates transfer of electrons from outside the cell membrane. (W. J. Ingledew, "Ferrous Ion Oxidation by Thiobacillus ferrooxidans" Biotechnology and Bioengineering Symposium No. 16, pp. 23-32, (1986).)
According to Ingledew, the iron-oxidase system of Thiobacillus ferrooxidans is a membrane bound enzyme complex that spans the cytoplasmic membrane of the organism. The Fe.sup.2+ oxidizing portion of the respiratory chain is short, consisting of four redox proteins: a blue colored rusticyanin (copper protein) and three cytochromes, a cytochrome oxidase, cytochrome c, and cytochrome a. (D. E. Rawlings and T. Kusano, "Molecular Genetics of Thiobacillus ferrooxidans" Microbiological Reviews 58 (1), pp. 39-55 (March, 1994).) The electron transfer components are organized in the cytoplasmic membrane in such a fashion as to couple Fe.sup.2 + oxidation to the generation of a transmembrane proton electrochemical potential (or proton-motive force) (.DELTA.P), measured to be 250 mV. A diagrammatic representation of this electron transfer mechanism is shown in FIG. 2, in which "out" and "in" refer to the bulk phase and the cytoplasm, respectively. In the iron-oxidase complex, the copper (Cu) protein rusticyanin is thought to be the initial electron acceptor from Fe.sup.2+. The midpoint potential (E.sub.m) of rusticyanin has been measured to be 680 mV at pH 3.2. During the growth of Thiobacillus ferrooxidans, the electrical potential (E.sub.b) of the Fe.sup.2+ /Fe.sup.3+ couple has been found to increase from approximately 555 mV to 800 mV as the Fe.sup.2+ is oxidized by the bacterial cells. The reduction of molecular oxygen is catalyzed by a cytochrome oxidase at a pH of 6.5 on the inside of the cytoplasmic membrane.
Enhancement of the catalytic systems utilized in processing sulfur compounds by Thiobacillus ferrooxidans is the subject of the inventor's prior patent, U.S. Pat. No. 5,508,014 issued Apr. 16, 1996 and incorporated herein by reference. The culture of Thiobacillus ferrooxidans utilized in that patent is deposited as ATCC #55720 (Budapest Treaty Deposit, American Type Culture Collection, 12301 Parklawn Drive, Rockville, Md.).